In SAS, an initiator device is typically a host such as a computer, and a target device is typically a storage device such as a mechanical hard disk drive (HDD) or solid-state disk drive (SSD). SAS uses a connection-based protocol as opposed to packet-based protocol. SAS link layer protocol provides the means for SAS initiators and targets to request connection paths to each other. When a connection is active, the connected initiator and target devices can exchange frames that contain commands, data and status information. SAS expanders act as connection-based switches and facilitate connecting multiple SAS initiator and target devices to each other.
The SAS Protocol Layer 4 (SPL-4) standard generally defines a physical link as two differential signal pairs, one pair in each direction, that connect two physical PHYs. In addition, the standard generally defines a connection as a temporary association between a SAS initiator port and a SAS target port using a pathway. The terms link and connection, as used herein, are to be understood as consonant with that definition.
FIG. 1 shows a simple example configuration in an SAS system 100, comprising an initiator 102 connected to an expander 104 over an SAS link 110. The system 100 further comprises one or more target devices 106, such as disk drives, over one or more links 112.
Serial SCSI storage systems have evolved over four generations, and a key value is that legacy generation devices can interoperate with the latest generation protocol and link rates. SAS is a differential pair interface with an end-to-end connection protocol. The electrical characteristics are also defined for the transmitter and receiver PHY and serializer-deserializer (SERDES). Supported bit rates for SAS are shown below in Table 1.
TABLE 1SASElectrical LinkRaw Data baudGenerationBit-raterateSERDES codingSAS-1 3.0 Gb/s2.4 Gb/s8b10b encodedSAS-2 6.0 Gb/s4.8 Gb/sSAS-312.0 Gb/s9.6 Gb/sSAS-422.5 Gb/s19.2 Gb/s 128b/150b packets
SAS natively makes a single end-to-end connection over an available electrical link or channel. This becomes very inefficient when a SAS-4 link is used for a SAS-2 connection, where bandwidth utilization is just 6 Gb/s from a 22.5 Gb/s lane.
Bandwidth utilization is a challenge for the latest SAS-4 (22.5 Gb/s), and is generally expected to be a challenge in future faster SAS generations. A storage infrastructure that comprises a diverse range of high capacity storage devices may not be able to individually saturate the maximum possible link rate speed. Most SAS hard disk drive (HDD) devices are not expected to support faster than 12 Gb/s links, while Serial ATA (SATA) hard disk drives support 6 Gb/s as their fastest possible link rates. This means that an unmodified SAS storage infrastructure will run at low bandwidth utilization rates when operating in connections to those devices (i.e., 25% for 6 GB/s devices, 50% for 12 Gb/s devices). Existing HDD media read/write rates are much lower than this at around 200 MB/s. An inability to leverage the full capacity of the available bandwidth of the technology generally results in suboptimum system performance.
Improvements in SAS links are desirable.